1. Field of the Invention
The present invention relates to a process for detecting upstream congestion within a distributed-architecture switch such as those used in packet-switched data transmission networks, known by the abbreviation ATM standing for xe2x80x9casynchronous transfer modexe2x80x9d.
2. Discussion of the Background
It applies in particular to the implementation within ATM networks of the category of service known by the abbreviation ABR standing for xe2x80x9cavailable bit ratexe2x80x9d. This service allows a user to use the passband available over the path of a previously established connection without obliging him/her to make a static reservation of resources when establishing the connection. In return for this flexibility, the two ends of the connection must adapt their bit rate to the passband available. For this purpose, particular ATM cells, known as RM cells, where RM stands for xe2x80x9cresource managementxe2x80x9d, make it possible to inform the ends of a connection about the availability of the resources in the network. These cells, generated and processed in real time by the end equipment and also by the ATM switches, comprise fields intended to convey bit rate information, and in particular the ER bit rate recommended to the source by the network, ER being the abbreviation for xe2x80x9cexplicit cell ratexe2x80x9d.
Switches which have this capabiity calculate the ER bit rate and write it to the RM cells. The various known algorithms for calculating the ER bit rate make it possible to share out the resources of a given output port among the various connections which pass through this port. These algorithms generally operate differently depending on whether the switch is in a state of congestion or not, that is to say whether the stream of cells bound for a relevant port is larger than the output bit rate permitted on this port or not.
Congestion is in principle excluded for categories of services which require a static reservation of the resources which they use. Moreover, the ATM cells corresponding to these categories of services are generally switched straightaway, without waiting. This is not the case for services which do not necessitate any reservation of resources, such as ABR, hence buffer memories are generally provided so as to absorb the excess cell rate, by waiting for the sources to reduce their rate in response to the network indications conveyed in the RM cells.
Under these conditions, the momentary congestion of an ouput port of a switch is manifested mechanically as an accumulation of cells bound for this port in the buffer memories. In the case of a distributed-architecture switch composed of several input junctions coupled together by a cross-connector, a momentary congestion of an output port is manifested as an accumulation in the input junctions of the cells bound for this port. This is referred to as upstream congestion. This distributing within the input junctions of the cells bound for a specified output port makes it difficult to aggregate the various partial items of information relating to the cells waiting in each junction, so as to allow bookkeeping operations on them.
The purpose of the invention is to solve this problem by defining the information to be managed in each input junction and by defining a means for integrating the partial items of information.
To this end, the subject of the invention is a process for detecting upstream congestion within a distributed-architecture ATM switch composed of several input and output junctions for transmission in an ATM cell network, including counting in each input junction the number of incoming cells and outgoing cells bound for a specified output port of a junction, transferring the results of the counts supplied by the input junctions to the output port, aggregating the results of the counts in the output junction, and declaring a congestion in the output port if the aggregate number of cells is greater than a specified threshold value.